A circuit known as a flyback converter is a switch mode power supply circuit commonly used in applications such as AC-to-DC wall adapter power supplies and battery chargers. FIG. 1 (Prior Art) is a block diagram of a simple flyback converter 1. Flyback converter 1 operates by repeatedly closing and opening a switch 2. Closing switch 2 causes a current 3 to flow from a first input node 4, through a primary 5 of a transformer 6, through the switch 2, and to a second input node 7. In one example, a rough DC voltage is present between the first and second input nodes 4 and 7. An alternating current (AC) line voltage may, for example, be rectified by a full wave bridge rectifier (not shown) and an associated smoothing capacitor (not shown) so that the rectified and smoothed rough DC voltage is present between the first and second input nodes 4 and 7.
When switch 2 is closed, the current 3 that flows through primary 5 causes energy to be stored in transformer 6. Switch 2 is then opened. When switch 2 is opened, energy stored in transformer 6 is transferred to the output of converter 1 in the form of a pulse of current 8 that flows through a secondary 9 of transformer 6 and through a diode 10. In FIG. 1, an output capacitor 11 is connected across output terminals 12 and 13 of the converter. The pulse of current 8 charges capacitor 11. In steady state operation in a constant voltage mode, switch 2 is switched to open and close rapidly and in such a manner that the output voltage VOUT on output capacitor 11 remains substantially constant.
Flyback converter 1 of FIG. 1 is considered in further detail. Flyback converter 1 includes a primary side 14 and a secondary side 15. Primary side 14 includes a primary side controller integrated circuit 16, the switch 2, and the primary 5 of transformer 6. Secondary side 15 includes the secondary 9 of transformer 6, an optocoupler 17, a secondary side constant voltage (CV) and constant current (CC) controller integrated circuit 18, the rectifying diode 10, the output capacitor 11, and a few other discrete components. Secondary side controller 18 is an integrated circuit packaged in an IC package with eight terminals.
In the constant-voltage (CV) operational mode, the output voltage VOUT across output terminals 12 and 13 is sensed by a resistor divider. The resistor divider includes resistor 19 and 20. The center tap 21 of the resistor divider is coupled to terminal CV− of integrated circuit 18 and within integrated circuit 18 to a non-inverting input lead of a voltage control amplifier 22. The voltage control amplifier 22 compares the voltage on the center tap of the divider input to a reference voltage VREF and outputs the result of the comparison onto terminal OUT1 of integrated circuit 18. If the result of the comparison causes the voltage on terminal OUT1 to be low, then a current 23 is pulled through the optocoupler 17. The current 23 flows through current limiting resistor 24, through optocoupler 17, through a blocking diode 25, and into terminal OUT1. When current 23 flows through optocoupler 17, the optocoupler 17 causes a corresponding current 26 to flow to the primary side controller 16. This current 26 is an error current that is indicative of the voltage level on output terminals 12 and 13. Primary side controller 16 receives the error current 26 and, based on the error current, controls the on/off duty cycle of switch 2 to regulate output voltage VOUT.
In the constant-current (CC) operational mode, the current being supplied by the power supply is sensed when it returns to secondary 9. The current, referred to as IOUT, is made to flow through a sense resistor 27. The voltage drop across sense resistor 27 is therefore indicative of the magnitude of the current IOUT. The voltage drop across sense resistor 27 is sensed by a constant current amplifier 28 within integrated circuit 18. If the voltage drop is greater than a predetermined value, then constant current amplifier 28 causes the voltage on terminal OUT2 to be low. If the voltage on terminal OUT2 is low, then a current 29 is pulled through current limiting resistor 24, optocoupler 17, and blocking diode 30. The current flow 29 through optocoupler 17 causes error current 26 to flow into primary side controller 16. Error current 26 is therefore indicative of the magnitude of the current IOUT. Based on error current 26, primary side controller 16 controls the on/off duty cycle of switch 2 to regulate output current IOUT. Flyback converter 1 either operates in the constant voltage mode or in the constant current mode, depending on the loading condition. In one example, if the IOUT output current through output terminals 12 and 13 would exceed a specified current, then converter 1 operates in the constant current mode, otherwise converter 1 operates in the constant voltage mode.
Secondary side controller integrated circuit 18 has eight terminals. There are two terminals CV− and CV+ for inputs to the constant voltage amplifier 22, and one terminal OUT1 for the output of the constant voltage amplifier 22. There are two terminals CC− and CC+ for inputs to the constant current amplifier 28, and one terminal OUT2 for the output of the constant current amplifier 28. The integrated circuit is powered via a power terminal VCC and is grounded via a ground terminal GND. Integrated circuit 18 has eight terminals.
There are numerous secondary side CV-CC controller integrated circuits on the market. FIG. 2 (Prior Art) is a block diagram of a flyback converter 51 that employs one such conventional secondary side CV-CC controller integrated circuit 52, the TSM1011 manufactured by STMicroelectronics. The illustration of the circuitry within integrated circuit 52 is based on assumptions, and is a simplification. For accurate detailed information, contact STMicroelectronics.
The circuit of FIG. 2 has a similar topology to the circuit of FIG. 1, except that the current sinking outputs of the two amplifiers 53 and 54 are both coupled to the same output terminal OPTO 55. This makes an ORing function which ensures that whenever the current or the voltage exceeds their respective CC and CV regulation values, IREG and VREG, the optocoupler 56 will be activated. Accordingly, if amplifier 53 senses an overvoltage condition, i.e., VOUT>VREG, then amplifier 53 sinks current into terminal 55, thereby pulling an error current through optocoupler 56 and generating an associated error current 57 back to the primary side controller 58. If amplifier 54 senses an overcurrent condition, i.e., IOUT>IREG, then amplifier 54 sinks an error current into terminal 55, thereby pulling an error current through optocoupler 56 and generating an associated error current 57 back to the primary side controller 58. Integrated circuit 52 is powered via VCC terminal 59 and has six terminals: VCC, GND, VCTL, ICTL, and OPTO. Although the circuit of FIG. 2 works well, further improvements and cost reductions are desired.